The need for cleaner burning fuels has resulted in a continuing world-wide effort to reduce organosulfur levels in hydrocarbon fluids containing such sulfur compounds such as gasoline and diesel fuels. The reduction of sulfur in these hydrocarbon containing fluids is considered a means for improving air quality because of the negative impact sulfur has on the performance of sulfur-sensitive items such as automotive catalytic converters. The presence of oxides of sulfur in automotive engine exhaust inhibits and can irreversibly poison noble metal catalysts in a converter. Emissions from an inefficient or poisoned converter contain levels of non-combusted, non-methane hydrocarbons, oxides of nitrogen, and carbon monoxide. Such emissions can be catalyzed by sunlight to form ground level ozone, more commonly referred to as smog.
Most of the sulfur in hydrocarbon-containing fluids, such as gasoline, comes from thermally processed gasolines. Thermally processed gasolines such as, for example, thermally cracked gasoline, visbreaker gasoline, coker gasoline and catalytically cracked gasoline (hereinafter collectively referred to as “cracked-gasoline”) contain, in part, olefins, aromatics, sulfur, and sulfur-containing compounds.
Since most gasolines, such as, for example automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and mixtures thereof contain a blend of, at least in part, cracked-gasoline, reduction of sulfur in cracked-gasoline will inherently serve to reduce sulfur levels in most gasolines.
Public discussion about gasoline sulfur has not centered on whether or not sulfur levels should be reduced. Rather, consensus has emerged that lower sulfur levels in gasoline can reduce automotive emissions and improve air quality. Thus, the real debate has focused on the required level of reduction, geographical areas in need of lower sulfur gasoline, and the time frame for implementation of lower sulfur levels.
As concern over the impact of automotive air pollution continues, it is clear that further efforts to reduce sulfur levels in automotive fuels will be required. While current gasoline products contain about 330 parts per million by weight (ppmw), the U.S. Environmental Protection Agency (USEPA) recently issued regulations requiring the average sulfur content in gasoline to be less than 30 ppm average with an 80 ppm cap. By 2006, the standards will effectively require every blend of gasoline sold in the United States to meet the 30 ppm level.
In addition to the need to be able to produce low sulfur content automotive fuels, there is also a need for a process which will have a minimal effect on the olefin content of such fuels so as to maintain the octane number (both research octane number (RON) and motor octane number (MON)). Such a process is desirable since saturation of olefins can greatly affect octane number. The adverse effect on olefin content is generally due to the severe conditions normally employed, such as during hydrodesulfurization, to remove thiophenic compounds (such as, for example, thiophenes, benzothiophenes, alkyl thiophenes, alkylbenzothiophenes, alkyl dibenzothiophenes, and the like) which are some of the most difficult sulfur-containing compounds to be removed from cracked-gasoline. In addition, there is a need to avoid a system wherein the conditions are such that the aromatic content of cracked-gasoline can be lost through saturation. Thus, there is a need for a process wherein desulfurization is achieved and the octane number is maintained.
In addition to a need for removal of sulfur from cracked-gasolines, there also is a need to reduce the sulfur content in diesel fuels. In removing sulfur from diesel fuels by hydrodesulfurization, the cetane is improved but there is a large cost in hydrogen consumption. Hydrogen is consumed by both hydrodesulfurization and aromatic hydrogenation reactions.
To satisfy these needs, processes for desulfurization of cracked-gasolines or diesel fuels have been developed, as disclosed in U.S. Pat. Nos. 6,254,766 and 6,274,533. These comprise contacting an organosulfur containing hydrocarbon stream with a sorbent in a desulfurization zone, separating the desulfurized hydrocarbon stream from the resulting sulfurized sorbent composition, regenerating at least a portion of the sulfurized sorbent composition to produce a regenerated, desulfurized sorbent composition, activating at least a portion of the regenerated desulfurized sorbent composition and thereafter using at least a portion of the activated, regenerated sorbent composition for further desulfurization of a selected hydrocarbon feed stock.
While such processes represent significant contributions to the art for the desulfurization of cracked-gasoline or diesel fuels in the providing a desulfurized product having low sulfur content, there is still an opportunity for improvements to such processes.
Since the volume of desulfurization sorbent employed in carrying out desulfurization processes can be significant when the processes are practiced on a commercial scale, such as the processing of cracked-gasolines or diesel fuels, it is highly desirable that the life of the sorbent be maximized to permit extended use in a desulfurization zone prior to subjecting the sulfurized sorbent to regeneration and activation.
Accordingly, it is an object of the present invention to provide an improved process for desulfurization of cracked-gasolines or diesel fuels when using sorbent compositions.
Another object of this invention is to provide a process for extending the useful life of sorbent compositions.
A further object of this invention is to provide a process for removal of sulfur from cracked-gasolines and diesel fuels which maximizes the useful life of sorbent compositions so as to extend its life in the desulfurization zone prior to its being regenerated and reactivated.